1. Field of the Invention
The present invention relates to a system and methods for recovering and regenerating extractive distillation (ED) solvent utilized to recover and purify the aromatic hydrocarbons from a petroleum stream. The invention further relates to a method for recovering and regenerating ED solvent from a petroleum stream containing at least a measurable amount of hydrocarbons which are heavier than the intended feedstock. The system and methods can effectively remove and recover the heavy hydrocarbons from a closed solvent loop through mild operating conditions with no additional process energy required.
2. Related Art
In recent history, distillation has continued to be one of the most vital processing steps in the separation of petroleum hydrocarbons and related compounds. These processing steps include specialized distillation processing procedures such as azeotropic distillation and extractive distillation.
Extractive distillation has been used on a greater scale in the chemical processing industry and is a key separation method used in chemical engineering. The separation sequence as employed by the various columns when combined with other separation processes, column and tower internals configurations and operational controls plays an important role in the design of extractive distillation.
One of the most important aspects of the process is the role of the solvent. Many different processes use conventional and novel separating agents such as a solid salt, liquid solvent, a combination of a solid salt and liquid solvent and ionic liquid are key components in the selection of a solvent. One key aspect of extractive distillation is that the separating agent, a solvent with a high boiling point, is added to a components mixture for separation which increases their relative volatility when the components have relatively similar or equal boiling points. For these extractive distillation schemes and processes, the selection of suitable solvent is fundamental to ensure an efficient, thoroughly effective and economical design.
In widespread industrial uses, ED is a commercially proficient unit operating in many applications in recent industrial and chemical processing. In this type of process, a nonvolatile polar solvent is added to the extractive distillation column (EDC) to increase the relative volatility between the polar and non-polar components or mixtures, which have similar or very close boiling points.
In general, the solvent is added to an upper section of the EDC, and the hydrocarbon feed or feedstock is introduced to a middle section of the EDC. On such example of the feedstock is benzene, toluene and xylene, typically referred to BTX aromatics, which are derived from the double hydrogenation of raw pyrolysis gasoline (RPG). As the nonvolatile solvent descend through the column, it preferentially extracts the polar components to form a rich solvent moving toward the lower or bottom section of the EDC. This allows the non-polar component vapor to ascend to the top of the column. The overhead vapor is condensed and a portion of the condensate is recycled to the top section of the EDC as reflux, while the other portion is withdrawn as the raffinate product.
The rich solvent containing the solvent and the polar components is fed into a solvent recovery column (SRC) to recover the polar components as an overhead product and the lean solvent (free of the feed components) as a bottom product, which is recycled to the upper portion of the EDC and is reused as the extractive solvent. A portion of the overhead product is recycled to the top of the SRC as the reflux to rectify and knock down any entrained solvent in the overhead vapor. The SRC is optionally operated under reduced pressure (typically a vacuum) or a stripping medium or both to reduce the column bottom temperature. This reduce pressure avoids solvent thermo-degradation due to high temperature at the bottom of the column.
Many different ED processes are described in the prior art and other literature, which are hereby incorporated by reference. Some exemplary references are as follows:
Aromatic/No-aromatic Separation: U.S. Pat. No. 7,078,580 to L. Tian, etal.; U.S. Pat. No. 4,053,369 to M. Cines; and F. Lee, et al., “TwoLiquid-Phase Extractive Distillation for Aromatics Recovery”, Ind. Eng.Chem. Res. (26) No. 3, 564-573, 1987.Diolefin/Olefin Separation: U.S. Pat. No. 4,269,668 to P. Patel.Cycloparaffin/Paraffin Separation: R. Brown, et al., “Way To PurifyCyclohexane”, Hydrocarbon Processing, 83-86, May 1991.
Recovering the aromatic hydrocarbons from the mixtures containing the aromatic and non-aromatic hydrocarbons can also be achieved by liquid-liquid extraction (LLE) or ED. Typical ED process configurations and their operations for aromatic hydrocarbon recovery are well known in the art. Most of the recent patents describing LLE distillation processes focus on the “solvent formulation” for improving the aromatics recovery from a particular aromatic feedstock. Although ED process requires less equipment (for example, only 2 instead of 4 separation columns) and have a lower energy requirement, as well as develop less problems during operation, the application of this process is more restricted by narrow boiling range of the feedstock than that of LLE process.
In order to achieve an acceptable aromatic purity and recovery, the solvent needs to keep essentially all benzene (the lightest aromatic compound; boiling at 80.1° C.) in the bottom section of the EDC, thus driving virtually all the heaviest non-aromatics into the overhead section of the EDC. For the narrow boiling-range (C6-C7) of aromatic feedstock, the heaviest non-aromatic compounds include ethylcyclopentane (boiling at 103.5° C.). For the full boiling-range (C6-C8) aromatic feedstock, the heaviest non-aromatic compounds include ethylcyclohexane (131.8° C.).
Therefore, it is much more difficult to recover BTX aromatics from the full boiling-range feedstock, such as the full range pyrolysis gasoline, than to recover benzene and toluene from the narrow boiling-range feedstock, such as the C6-C7 reformate. An ED process which is suitable for the narrow boiling-range aromatic feedstock, may not be able to satisfactorily process the full boiling-range aromatic feedstock.
Another critical problem of the ED process for use in aromatics recovery, which may cause the process to fail or have great inefficiencies, is the existence of measurable amount of heavy (C9-C12) hydrocarbons in the ED feedstock, especially for the process for recovering BTX aromatics from the full boiling-range (C6-C8) feedstock. This critical problem has been recognized or discussed in the prior art, probably due to the fact that, up to this point, there are no commercial ED plants for processing full boiling-range aromatic feedstock, such as the full boiling-range (C6-C8) pyrolysis gasoline being reported in operation.
In both ED and LLE processes for aromatics recovery, the solvent is circulated in the process system indefinitely in a closed loop. Normally, the ED or LLE feedstock is fed to a prefractionator for removing the heavy portion of solvent, leaving only the desirable portion to be fed to the EDC or LLE column. Under reasonable operating conditions, even a well designed prefractionator unavoidably slips some measurable amount of heavy hydrocarbons into the feed stream feeding to the EDC or LLE process column. The heavy hydrocarbons in the feed stream would significantly increase under a poorly designed, operated or malfunctioned prefractionator.
To remove the heavy hydrocarbons and the polymerized heavy materials derivate from oxidized solvent, commercial LLE processes use a solvent regenerator where a small slip stream of the lean solvent (approximately one (1%) percent of the lean solvent stream) is heated with or without stripping steam to recover the regenerated solvent or any other heavy components having boiling points which are lower than that of the solvent. The heavy polymeric materials having a boiling point higher than that of the solvent are removed from the bottom of the solvent regenerator as sludge.
G. Asselin U.S. Pat. No. 4,048,062 discloses the LLE process scheme for aromatics recovery, in which a portion of lean solvent (virtually free from hydrocarbons) from the bottom of the SRC is introduced into a solvent regeneration zone. A vaporous stripping medium is also introduced into the solvent regeneration zone separately, recovered with regenerated solvent and introduced into the SRC as at least a portion of the total vaporous stripping medium. The extraction solvent is sulfolane and water mixture. The vaporous stripping medium is steam.
Sulfolane (2,3,4,5-tetrahydrothiophene-1,1-dioxide) is a clear, colorless liquid commonly used in the chemical industry as an extractive distillation solvent or reaction solvent. Sulfolane was originally developed in the 1960s as a solvent to purify butadiene. Sulfolane is an aprotic organosulfur compound, and it is readily soluble in water.
Over the years, the solvent regeneration scheme disclosed in U.S. Pat. No. 4,048,062 has been successfully demonstrated in many Universal Oil Products (UOP) or Institut Français du Pétrole (IFP) designed commercial LLE processes for aromatics recovery using sulfolane and water as the extractive solvent. This is because most of the measurable amount of heavy (C9 to C12) hydrocarbons in the feedstock are rejected by the solvent phase in the LLE column and removed with the raffinate phase as a part of the non-aromatic product.
In normal EDC operation for aromatics recovery, however, these heavy hydrocarbons tend to stay with the rich solvent at the bottom of the EDC due to their high boiling points. Even for the narrow boiling-range (C6-C7) feedstock, there can be a measurable amount of heavy (C9+) hydrocarbons trapped in the solvent, most of which can only be removed from the solvent by increasing the temperature, vacuum level, and stripping steam of the SRC. This method is not a desirable or preferred method of removing the hydrocarbons trapped in the solvent. For the full boiling-range (C6-C8) feed, however, the boiling points of the heavy hydrocarbons, are too high to be stripped from the solvent in the SRC.
Therefore, if these heavy hydrocarbons are not removed from the feed in the upstream prefractionation column, the heavy hydrocarbons will continuously accumulate in the solvent and cause inefficient column and process operation. This is due to the solvent being circulated between the EDC and the SRC indefinitely within a closed loop.
The solvent regeneration scheme disclosed in U.S. Pat. No. 4,048,062, is no longer adequate for the ED process, since it was designed for the LLE process for removing minor amounts of polymeric materials which may possibly be formed from the reactions between the oxidized or decomposed solvent components and trace amounts of the heavy hydrocarbons in the solvent. Using this solvent regeneration scheme in the ED process, the heavy hydrocarbons continuously accumulate and polymerize in the closed solvent loop, until the polymerized materials have boiling points higher that of sulfolane (>287° C.) before they can get out of the closed loop through the bottom of the solvent regenerator. It is a potentially disastrous situation since excessive polymeric materials in the solvent can not only significantly change the solvent properties (more particularly, the selectivity and solvency of the solvent), but can also cause plugging in process equipment, such as pumps, valves, column internals, lines, etc., reducing the efficiency of the system and plant and eventually rendering the ED process inoperable.
In a typical prior art ED process, the aromatic and non-aromatic hydrocarbon feed mixture is introduced into a middle section of the EDC. A water-soluble solvent is more characteristically selective for absorbing the more polar hydrocarbons introduced into an upper section (above the hydrocarbon feed entry point) of the EDC. A rich solvent (extract) stream is recovered from a bottom section of the EDC. The rich solvent (extract) stream contains aromatic hydrocarbons, a measurable amount of heavy (C9+) hydrocarbons and the water-soluble solvent.
A solvent-free raffinate stream containing non-aromatic hydrocarbons and water is recovered from the top or upper section of the EDC. The overhead raffinate stream is introduced into an overhead receiver, which serves to effect a phase separation between the non-aromatic hydrocarbons and the water phases. A portion of the non-aromatic hydrocarbon phase is recycled to the top section of the EDC as the reflux, while the other portion is withdrawn as the non-aromatic hydrocarbon product. The water phase is then transferred to the steam generator to generate a stripping steam for the SRC and the solvent regenerator, if required.
The rich solvent stream which is removed from the bottom section of the EDC is introduced into a middle section of the SRC, and the stripping steam is injected into a lower section of the SRC to facilitate the removal of the aromatic hydrocarbons from the solvent. An aromatic concentrate, containing water and being substantially free from solvent and non-aromatic hydrocarbons, is withdrawn as an overhead stream from the SRC and introduced into an overhead receiver. The overhead receiver serves to effect a phase separation between the aromatic hydrocarbons phase, which are recovered, and the water phase.
A portion of the aromatic hydrocarbon phase is recycled to a top section of the SRC as the reflux, while the other portion is withdrawn as the aromatic hydrocarbon product. The water phase is transferred to the steam generator to generate the stripping steam for the SRC and the solvent regenerator, if required.
A lean solvent stream containing a measurable amount of heavy (C9-C12) hydrocarbons is withdrawn from the bottom section of the SRC. The greater proportion thereof is recycled to the upper section of the EDC as the lean solvent feed.
A portion of the lean solvent is diverted and introduced into a solvent regenerator and, optionally, a vaporous stripping medium (steam) is introduced into the solvent regenerator, through an entry point below the lean solvent feed entry point. The stripping medium (steam) supplied by the steam generator in admixture with regenerated solvent is then introduced into the SRC as at least a portion of the stripping steam for the SRC.
In order to minimize the bottom temperature of the solvent regenerator, the solvent regenerator is operated preferably under the same reduced pressure (vacuum) as the pressure in the SRC Regenerated solvent containing heavy materials (with boiling points below the solvent boiling point) and substantially all the steam, is recovered as an overhead stream of the solvent regenerator and introduced into the bottom or lower section of the SRC as a part of the stripping steam, or if stripping steam is not used, mixed directly into the lean solvent from the bottom section of the SRC. Deteriorated solvent products and polymeric sludge are removed as a bottom stream of the solvent regenerator.
Degradation of the sulfolane solvent occurs at temperatures above 200° C. in an inert atmosphere and increases significantly in oxygen-containing atmosphere, where air leaks through the ED process equipment. The primary products from degradation of sulfolane are sulfur dioxide and oxygen-containing organic compounds, such as aldehydes, organosulfonic acids, carboxylic acids, etc.
Further interactions among the primary products can lead to undesirable polymeric materials and sludge. To prevent corrosion of the process equipment, monoethanolamine (MEA) is normally used to neutralize acidic materials and adjust the pH value of the sulfolane solvent. The heavy salts produced from acid neutralization along with other solvent additives, such as antifoaming agents, are also removed as a part of the sludge from bottom of the solvent regenerator.
It would be desirable to have a system and methods which reduces the sludge and plugging in the system. The system and methods would operate a similar conditions through all the components of the system and recover the solvent in an efficient method. The system and methods would be easy to operate and control with a reducing in plant energy requirements, conserve raw materials and be easier to maintain.